It is often desirable for a photosensor array to contain at least one photosensor capable of operating under a variety of different lighting conditions (e.g., high light levels) and poorly lit conditions (e.g., low light levels). This feature is typically provided by changing the exposure time of the photosensor. For example, under bright lighting conditions, the exposure time of the photosensor is reduced to prevent the photosensor from saturating.
Since the present invention relates to the response of a photosensor under different lighting conditions, it is useful at this point to briefly review a number of the operating characteristics of photosensors in reference to FIGS. 1 and 2.
The full well capacity of a photosensor array refers to the total amount of charge that can be stored in any one of its photosensors before overflowing into adjoining photosensors. Accordingly, the full well capacity of a photosensor array is dependent upon the physical size of its photosensors.
Dynamic range is defined as the maximum signal strength achievable by a photosensor array divided by the noise in the array. The maximum achievable signal strength by a photosensor array is determined by the full well capacity of the array. The noise in the photosensor array is the sum of dark and read noise components. Accordingly, the dynamic range of a photosensor array can be described by equation (1) below.
                              Dynamic          ⁢                                          ⁢          Range                =                              Full            ⁢                                                  ⁢            well            ⁢                                                  ⁢            capacity                                              Dark              ⁢                                                          ⁢              Noise                        +                          Read              ⁢                                                          ⁢              Noise                                                          (        1        )            
Thus, one way of increasing the dynamic range of a photosensor array is to increase its full well capacity.
Responsivity is a measure of the effectiveness of a photosensor in converting incident electromagnetic radiation into electrical current or voltage, and is inversely related to the capacitance of the photosensor.
There are two main sources of noise in a photosensor array, namely photon shot noise and read noise.
Photon shot noise results from natural fluctuations in the number of photons detected by a photosensor, and is caused by the quantum statistical nature of photon emission and detection. Thus, photon shot noise imposes a fundamental limit on the responsivity of a photosensor array insofar as it determines the minimum noise level achievable therein.
Photon shot noise is governed by Poisson statistics and is described by the square root of the flux (F) of radiation incident on the photosensor (e.g., 1M photogenerated electrons corresponds with a photon shot noise of √{square root over (106)}, or 103, and a signal to noise ratio (SNR) of 103).
From the above, it can be seen that increasing flux density (F) reduces the relative fraction of photon shot noise. However, as will be recalled, the full well capacity of a photosensor array provides an upper limit on the number of photons that can be integrated in its photosensors. Thus, any attempt to increase the flux density of the radiation incident on a photosensor and the detection thereof (to reduce the relative effect of photon shot noise) needs to be accompanied by an increase in the full well capacity of the photosensor array. This is typically achieved by including a large storage capacitor in each photosensor.
However, having a large full well capacity can present problems at low light levels. In particular, since the voltage output (V) of a photosensor is inversely related to its capacitance (C) (i.e., V=Q/C), the inclusion of a large capacitance in a photosensor reduces the output voltage swing from the photosensor.
Read noise is a combination of system noise components inherent in the conversion of photogenerated charge carriers into a measurable signal; processing of the resulting signal; and analog-to-digital (A/D) conversion thereof.
Thus, read noise determines the lower detection limit of a photosensor. Unfortunately, one of the major components of read noise is reset noise, which depends on the capacitance of the photosensor. Accordingly, any attempt to reduce the read noise of a photosensor (by reducing its full well capacity) conflicts with increasing dynamic range and reducing photon shot noise.
U.S. Pat. No. 5,296,698 describes a photosensing device that uses the “varicap” effect to vary the response of a (single) photodiode by changing the voltage applied across its PN junction. However, the photosensing device in U.S. Pat. No. 5,296,698 is a lateral device, which is not suitable for use in photosensor arrays.
Referring to FIG. 1, in a 3T photosensor 10, photogenerated charge is collected by photodiode 12 and converted to a voltage (Vpd) on the gate of a sense transistor M1. Increasing (or decreasing) the light-sensitive area of the photosensor 10 increases (or decreases) the photogenerated charge collected therein, and the photosensor's capacitance at the same rate. Hence, the voltage swing in a 3T photosensor 10 remains constant.
Referring now to FIG. 2, a 4T photosensor 14 separates the photocharge collection and sensing areas. In particular, the 4T photosensor 14 relies on a charge transfer from the charge collection capacitor Cpd to the sensing capacitor Csn. While this approach works well for small (<5 μm) photosensors, it is not as effective for larger (≧30 μm) photosensors.
U.S. Pat. No. 6,801,258 describes a photosensor array that is particularly suited for detection at low light levels. Under these conditions, the dominant source of noise in the photosensor array is read noise. However, averaging N signals from the array results in a √{square root over (N)} reduction of its noise.
In summary, prior art photosensors are characterized by having either a low capacitance or a high capacitance. Photosensors with a low capacitance typically have high sensitivity but increased photon shot noise. In contrast, photosensors with a high capacitance typically have low photon shot noise but low sensitivity. However, the lower sensitivity of such photosensors means that read noise provides a more significant contribution to the overall noise of the photosensor. Consequently, the relative noise level of the photosensor is increased.